All Players
Got to the cheats screen and enter:
OMGTKKYB
now in head to head mode all the characters will be unlocked

Master cheat:
Select the "Enter Cheat" option from the main menu. Enter ZBOBBYBIRD as a code to unlock all cheat features (all multi-player characters, invincibility, unlimited ammunition, all weapons, and level select) under the "Cheats" option. The name of the cheat will appear to confirm correct code entry. Note: Any cheats that are enabled can be saved during game play.

Chef in multi-player mode:
Enter YLOVEMACHINE as a code.

Wendy in multi-player mode:
Enter BCHECKATACO as a code.

Terrence in multi-player mode:
Enter SRAFT as a code.

Phillip in multi-player mode:
Enter PPHAERT as a code.

Ned in multi-player mode:
Enter JHAWKING as a code.

Mr. Macky in multi-player mode:
Enter ACHEATINGSBAD as a code.

Officer Barbrady in multi-player mode:
Enter DELVISLIVES as a code.

Big Gay Al in multi-player mode:
Enter GOUTRANGE as a code.

Starvin Marvin in multi-player mode:
Enter MSLAPUPMEAL as a code.

Mr. Garrison in multi-player mode:
Enter VDOROTHYSFRIEND as a code.

Pip in multi-player mode:
Enter EFISHNCHIPS as a code.

Jimbo in multi-player mode:
Enter QSTARINGFROG as a code.

Ike in multi-player mode:
Enter HKICKME as a code.

Ms. Cartman in multi-player mode:
Enter KALLWOMAN as a code.

Mephisto in multi-player mode:
Enter NGOODSCIENCE as a code.

Alien Visitor in multi-player mode:
Enter TMAJESTIC as a code.

Hint: Different weapons:
While playing any level you can change certain weapons to do different things. For example, press Select while using the dodgeball to change it to the super dodgeball. Press Select when using the snowball to get yellow snow. Press Select when using the warpo ray to get the piranha gun. Press Select when using the sponge dart gun press to get the super sponge dart gun. Note: This trick has no effect with the launcher or the chicken sniper rifle.